1. Field of the Invention
The invention relates to a process for preparing siloxanes from alkali metal salts of organosilanols (also referred to hereinafter as siliconates) wherein the molar ratio of alkali metal cation to silicon is <1 with halosilanes.
2. Description of the Related Art
Defined cyclic siloxanes of the general formula (1)[RSi(OSiR1R2R3)O]n  (1)are of great industrial interest as, for instance, highly reactive crosslinkers for optical, electronic, and high-temperature applications, as thermally stable solvents, as complexing agents, as fluids for cosmetic applications, and as potential building blocks for siloxane polymers. To date they have been very costly and inconvenient to produce, thus making their production uneconomical and unsuited to production on an industrial scale.
U.S. Pat. No. 2,567,110 (Corning Glass, 1947) describes very generally a process for preparing organosiloxanes by reaction of alkali metal salts of organosilanols with chlorosilanes. In the description there are indeed cyclic structures for salts of organosilanols (RSiOOAlkali)n, but there are no cyclic siloxanes of the structure (RSiOSiR′3O)n derivable therefrom. In the salts of the mono-organosilanols, the alkali:Si ratio is 1, 2 or 3 in each case. In the sole examples with monoorganosilanol salts (ex. 1: sodium methylsiliconate, ex. 2 sodium phenylsiliconate), the reactions with trimethylchlorosilane in a solvent mixture lead in each case to high molecular mass polysiloxanes.
JP 2013-241497 (Nat. Univ. Corp. Gunma Univ., 2012) describes the preparation of sodium or potassium siliconates starting from C2-C8 trialkoxysilanes and sodium hydroxide or potassium hydroxide solution in a solvent. Solid products are obtained by filtration and drying of the filter cake. The alkali:Si ratio here is 1:1. These siliconates are reacted subsequently with chlorosilanes (e.g., HM2-silane) in the presence of solvents and pyridine to give the corresponding cyclosiloxanes (e.g., [MeSi(OSiMe2H)O]4).
According to Shchegolikhina, O. I. et al., European Journal of Inorganic Chemistry, 2004, 1253-1261, cyclic sodium and potassium phenylsiloxanolates are prepared from phenyltriethoxysilane with aqueous sodium or potassium hydroxide solution, optionally in the presence of alcohols and organic nonpolar solvents. The alkali/Si ratio in each case is 1:1. The reaction products are reacted in the presence of pyridine with trimethylchlorosilane to give the corresponding cyclic derivatives (cis-[PhSi(OSiMe3)O]4 or 3).
Shchegolikhina, O. I. et al., Russian Chemical Bulletin, International Edition, Vol. 56, No. 1, pp. 83-90, January 2007, describes siliconate salts having an alkali:Si ratio of 1:1 as starting materials for the target compounds. They are converted into corresponding cyclic siloxanes using a large excess (>8 eq. relative to siliconate) of trimethylchlorosilane in the presence of pyridine as a base in high dilution in hexane as solvent. The alcohol bound in the crystal, and any additional water from the preparation of the siliconate, consumes a part of the chlorosilane, thus reducing the economics of the operation.
According to Laine, R. M et al., Comptes Rendus Chimie 13, 270-281, 2010, octaphenylsilsesquioxane, for example, is cleaved to give the corresponding cyclic sodium siloxanolate within 24-48 h using an excess of NaOH in butanol. The siloxanolate is reacted with methyltrichlorosilane in methanol to give a dimethoxysilyl derivative, which can be subjected to hydrolysis/condensation to form an asymmetrical silsesquioxane (“Janus silsesquioxane cube”).
DE 602004010461 T2 (Samsung Electronics Co., 2004) includes in its description polyfunctional cyclic siloxane compounds such as, for example, [MeSiO(SiMe2Cl)O]4 and [MeSiO(SiMe2-OMe)O]4.
In accordance with the prior art cited above, cyclic products of the general formula (1)[RSi(OSiR1R2R3)O]n  (1)are accessible from the corresponding alkali metal siliconates of the general formula (2a)RSi(OH)2OM  (2a)where M is an alkali metal, R is an organic radical, and n is 3, 4, 5, 6, 7 or 8, or from their condensation products, by reaction with halosilanes of the general formula (3)R1R2R3Si-Hal  (3)where Hal is a halogen radical and R1, R2, and R3 independently of one another are a hydrogen radical, a halogen radical or an organic radical optionally bonded via oxygen. The alkali:silicon ratio (M:Si) here in the general formula (2a) is always 1:1.
These siliconates are obtained by reaction either of one mole equivalent of alkoxysilane of the general formula (4)RSi(OR′)3  (4)or its hydrolysate with one mole equivalent of aqueous alkali metal hydroxide. R has the definition above and R′ in formula (4) is a lower alkyl radical.
As a result of the high alkali metal content of the siliconate, complete conversion to the target product of the general formula (1) produces one mole equivalent of alkali metal salt per mole equivalent of siliconate, the salt having to be recycled or disposed of, which is expensive. A further disadvantage arising from the high alkali metal content is the hygroscopicity of salts, which increases with rising alkali metal content, and which means that the siliconates used according to the prior art are difficult to free from alcohol or water of hydration that is bound in the crystal or is adhering (see Shchegolikhina, O. I. et al., European Journal of Inorganic Chemistry, 2004, 1253-1261). For economic reasons, however, it is necessary to employ a water-free and alcohol-free siliconate in the reaction with the halosilane of the general formula (3), since otherwise a portion of the halosilane is consumed for the reaction with the OH-functional secondary constituents, and is not available for the desired reaction with the siliconate. If drying is not carried out completely, it is necessary, for maximum conversion of the water-moist and alcohol-moist siliconate into the target product of the general formula (1), to use an excess of halosilane of the general formula (3), but this is uneconomical, because it leads to unwanted secondary products which require disposal. Another aspect is the decomposition tendency of the siliconates, which likewise shifts toward lower temperatures as the alkali metal content goes up, with a prolongation of the drying times (cf. WO2012/022544). The processes described to date in the technical literature for preparing the siloxanes of the general formula (1) according to the invention, by reaction of the dried siliconates of the general formula (2a) with halosilanes of the general formula (3), always use a solvent. After the reaction, however, these solvents have to be removed again, in a costly and inconvenient procedure, from the target products of the general formula (1).